Fabrication Methods for Micro Compound Optics

ABSTRACT

Methods for fabricating refractive element(s) and aligning the elements in a compound optic, typically to a zone plate element. The techniques are used for fabricating micro refractive, such as Fresnel, optics and compound optics including two or more optical elements for short wavelength radiation. One application is the fabrication of the Achromatic Fresnel Optic (AFO). Techniques for fabricating the refractive element generally include: 1) ultra-high precision mechanical machining, e.g,. diamond turning; 2) lithographic techniques including gray-scale lithography and multi-step lithographic processes; 3) high-energy beam machining, such as electron-beam, focused ion beam, laser, and plasma-beam machining; and 4) photo-induced chemical etching techniques. Also addressed are methods of aligning the two optical elements during fabrication and methods of maintaining the alignment during subsequent operation.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/688,187, filed on Oct. 17, 2003, which claims the benefit under 35USC 119(e) of U.S. Provisional Application No. 60/419,331 filed on Oct.17, 2002, both of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

The present invention pertains generally to fabrication techniques to beused for fabricating micro refractive, such as Fresnel, optics andcompound optic comprising two or optical elements for short wavelengthradiation. One application is the fabrication of the (AFO) described inU.S. patent application Ser. No. 10/134,026, which is incorporatedherein by this reference in its entirety.

The Achromatic Fresnel Optic (AFO) is a multi, such as two, elementcompound optic that is comprised of a diffractive Fresnel zone plate anda one or more refractive Fresnel lenses. The optic is described in U.S.patent application Ser. No. 10/134,026, (U.S. Pat. Application.Publication No. US 2005/0168820 A1) which is incorporated herein by thisreference in its entirety. Further uses for the optic are described inU.S. patent application Ser. No. 10/683,872 filed on Oct. 10, 2003, byWenbing Yun and Yuxin Wang, (U.S. Pat. Application Publication No. US2004/0165165 A1), which is incorporated herein by this reference in itsentirety.

Generally, the AFO is used for imaging short wavelength radiationincluding extreme ultraviolet (EUV) and x-ray radiation with wavelengthsin the range of 0.02 nanometers (nm) to 20 nm. The diffractiveelement(s) is the primary focusing element, and the refractive elementtypically provides no or very little net focusing effect. It serves tocorrect the chromatic aberration of the zone plate.

SUMMARY OF THE INVENTION

The techniques for fabricating the zone plate element are well known inthe art. They include, photo and electron-beam lithography techniques,and sputter-slice techniques. Challenges arise, however, whenfabricating compound optics and Fresnel refractive optics for theseshort wavelength radiation applications.

Generally, the present invention describes methods of fabricating therefractive element(s) and aligning the elements in the compound opticand thus to the zone plate element. More specifically, the inventionconcerns the techniques that are used for fabricating micro refractive,such as Fresnel, optics and compound optics comprising two or opticalelements for short wavelength radiation. One application is thefabrication of the Achromatic Fresnel Optic (AFO).

Techniques for fabricating the refractive element generally include: 1)ultra-high precision mechanical machining, e.g,. diamond turning; 2)lithographic techniques including gray-scale lithography and multi-steplithographic processes; 3) high-energy beam machining, such aselectron-beam, focused ion beam, laser, and plasma-beam machining; and4) photo-induced chemical etching techniques. Also addressed are methodsof aligning the two optical elements during fabrication and methods ofmaintaining the alignment during subsequent operation.

In general according to one aspect, the invention features a method forfabricating a compound optic for short wavelength radiation. The methodcomprises removing material of a substrate to form a surface profile ofa first optical element of the compound optic. This can be performedmechanically or chemically. The second optical element of the compoundoptic is also formed on the substrate.

In general according to another aspect, the invention features a methodfor fabricating a compound optic for short wavelength radiation. Themethod comprises forming a surface profile of a first optical element ofthe compound optic on a substrate, while also forming a fiducial mark onthe substrate. The second optical element of the compound optic is thenformed by reference to the fiducial mark.

In general according to still another aspect, the invention features anoptical element for short wavelength radiation. The element comprisesconcentric rings for focusing a beam of short wavelength radiation andsegments extending at least partially radially between the concentricrings to support the rings.

A frame is also preferably provided. It extends around at least aportion of a perimeter of the concentric rings, with the segmentsextending between the rings and the frame to support the rings in theframe.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a side plan, cross sectional view of a compound optic or AFO;

FIGS. 2A and 2B are side plan, cross sectional views illustrating thefabrication of the refractive element using mechanical removal ofmaterial on the substrate;

FIGS. 3A-3C are side plan, cross sectional views illustrating agray-scale lithography method for the fabrication of the refractiveelement;

FIGS. 4A and 4B are side plan, cross sectional views illustrating amulti-step process for forming the refractive element;

FIGS. 5A and 5B are side plan, cross sectional views illustrating aprocess for forming the refractive element using a high energy beam 50;

FIGS. 6A-6C are side plan, cross sectional views illustrating a processfor forming the refractive element using photo-induced chemical etching;

FIGS. 7A-7C are side plan, cross sectional views illustrating a processfor aligning the diffractive zone plate element with the refractiveFresnel lens element;

FIG. 8 is a side plan, cross sectional view illustrating a process forfabricating the refractive Fresnel lens element using a hybridfabrication solution; and

FIG. 9 is a schematic plan view of a free standing zone plate lenselement according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a compound optic or AFO 1, to which the present inventionis applicable in one example.

The exemplary compound optic 1 comprises a diffractive Fresnel zoneplate element 5 and one or more refractive Fresnel lens elements 6.

The compound optic 1 is shown fabricated on a single substrate 8. Inpractice, the different elements 5, 6 can also be fabricated onseparated substrates, in other embodiments.

In the example in which the compound optic 1 is an AFO, it includes aprimary focusing element, which is the diffractive Fresnel zone plate 5,and chromatic dispersion compensating elements which is the refractivelenses 6. The refractive lens 6 compensates for the chromatic dispersionof the zone plate 5 but with no or very small focusing effect.

For micro-imaging applications involving short wavelength radiation,i.e., radiation in the wavelength range of 0.02 nanometers (nm) to 20nm, the width of the segments in the refractive lens 6 typically rangefrom many millimeters in the center segments 110 to below 1 micrometernear the edge segments 112. The profile accuracy required is about 10 nmand less. We will describe five methods that can be used to fabricatethe required segments.

Ultra-High Precision Mechanical Machining

This method involves mechanical removal of material on a substrate inorder to produce the desired lens profile.

FIGS. 2A and 2B illustrate the fabrication of the refractive element 6on an unpatterned substrate 8.

Specifically, FIG. 2A shows a sharp single-crystal diamond tool tip 10of a diamond turning machine. The diamond tool tip 10 is controlled by aprecision positioning system 11 and is driven along the surface 114 ofthe substrate 8. In one example, the substrate is silicon wafer materialor copper. The tool 10 removes the material of the substrate 8 typicallywhile the substrate is turned or rotated around a center axis 116 tothereby perform the cut.

As shown in FIG. 2B, once the surface profile 110, 112 is machined, thesubstrate 8 is typically thinned from the backside by removing thematerial in region 12 to thereby form an optical port to increase thetransmission, if necessary. Finally, the second optical element, such asa diffractive zone plate element is then formed in the optical port 12,in one example.

The precision of diamond machining tools can be as high as 10 nm and areable to machine most materials required for the refractive lens, such assilicon and copper.

Lithographic Fabrication

This method involves patterning a photoresist, then developing theresist, and transferring the profile of the developed resist to thesubstrate 8. Two methods can be used with this technique.

FIGS. 3A-3C show a gray-scale lithography method.

As shown in FIG. 3A, the substrate 8 is first coated with a layer ofphotoresist 14. Then, the photoresist 14 is exposed with a spatiallyvarying dosage (see dosage exposure profile 18) that corresponds to theinverse of the desired surface profile.

Various types of exposure beams 16 can be used. Typically, the exposurebeam is visible light, ultraviolet light, x-ray radiation, electrons andions.

As shown in FIG. 3B, the resist 14 is then developed, yielding a profile110, 112 similar to the desired surface pattern.

As shown in FIG. 3C, the substrate 8 is then etched in the transferetching step to produce the desired surface profile in the substrate.

It should be noted that the response of the photoresist 14 in exposure,development, and the transfer etching is non-linear. Therefore, carefulcalibration is required for high yields. This technique can produceresolution as high as tens of nanometers.

Finally, the diffractive element 5 formed on the backside of thesubstrate 8.

FIGS. 4A and 4B show an alternative method, using a multi-step processsuch as those used in semiconductor fabrication.

Here the smooth profile of the lens 6 needs to be approximated by astaircase pattern as shown in the FIG. 4A inset.

This is fabricated according to the following process as shown in FIG.4B. A substrate 8 is first coated with a first layer of silicon 20 andthen a layer of photoresist 22. An etch stop layer is typically locatedbetween the silicon layer 20 and the substrate 8. The photoresist 22 isexposed with a pattern (see exposure dosage profile 24) that correspondsto the lowest level of the staircase. After the resist 22 is developed,the first silicon layer 20 is etched to yield the lowest part of thestaircase. The lens/substrate is then coated with, possibly, a thin etchstop layer and then resist 25 and polished to produce a flat surface,and then coated with another layer of silicon 26. Another layer ofphotoresist 28 is coated over this silicon layer 26 and exposed with apattern that corresponds to the next level of the staircase (seeexposure dosage profile 30). A two-level staircase pattern will beproduced after the resisted is developed and the silicon layer is etched(see reference numeral 60). This process is repeated until the desiredstaircase profile is obtained (see reference 62). The result pattern isencased in photoresist, and removing the photoresist will produce therefractive lens (see reference 64). The substrate 8 can be thinned orremoved to reduce absorption.

Finally, the diffractive element 5 formed on the backside of thesubstrate 8.

High-Energy Beam Fabrication

FIGS. 5A and 5B show a method in which a high energy beam 50 of, but notlimited to, laser, electron, ion, and plasma is used to ablate materialon the substrate.

Specifically, as shown in FIG. 5A, high energy beam 50 is directed andscanned over the substrate 8. Typically the beam 50 is a laser,electron, ion, and plasma beam that ablates material on the substratesurface. The relative movement between the beam and the substrate 8 iscontrolled, sometimes by rotating the substrate around its center axisin order to produce the desired profile 110, 112 as shown in FIG. 5B.

This method is analogous to the diamond-turning machine in that the lensprofile is produced directly on the substrate 8 in a 1-step process,except that energetic particles are used instead of a solid tool tip.This method can achieve about 1 micrometer accuracy with lasers andbetter than 10 nm accuracy with a focused ion beam. The substrate 8 ofthe finished lens can be thinned from the back to reduce absorption, andthe diffractive element 5 formed on the backside.

In the fabrication of the Fresnel refractive lens, the micromachiningtools, such as focused ion beam milling, may need to be calibrated tofabricate the Fresnel lens with accurate linear dimensions, accuratedepth profile, and without distortions.

For calibration, features, preferably linear scales 128, are firstfabricated on the substrate 8 by a suitable, well-calibrated process.One such process is electron beam lithography, which is well understood.Features produced by the micromachining tool, such as 110, 112 arecompared with the calibration features 128 to control and correct thecalibration of the tool during the fabrication of those features.

Calibration features 128 in the form of linear scales in the plane ofthe lens or as trenches with an accurate depth profile for depthdetermination are preferably used.

Photo-Induced Chemical Etching

FIGS. 6A to 6C show a method for forming the refractive element 6 thattakes advantage of the property that the etching rate of certainmaterials is dramatically increased when heated or in liquid state. As anon-limiting example, we assume a refractive lens 6 made of silicon.

Referring to FIG. 6A, the silicon substrate 8 is placed in a chlorinegas environment 118. A high-power laser spot 50 is then focused onto thesurface of the silicon wafer 8 causing the surface to locally heat upand melt into a molten state. This causes the reaction rate withchlorine to increase twenty fold, and the molten zone is etched away ata much higher rate than the unheated region to yield the desired profile110, 112 as shown in FIG. 6B. This method is capable of producingfeatures with up to 1 micrometer μm accuracy in the transverse directionand 10 nm in the longitudinal direction.

As shown in FIG. 6C, backside thinning is further performed in someimplementations to produce an optical port 72 to improve transmission.

Alignment of the Lens Elements of the Compound Optic

FIGS. 7A-7C shows the alignment of the lens elements of the compoundoptic 1.

As shown in FIG. 7A, the process begins typically with the first,refractive lens element 6, which has been fabricated according to one ofthe previously defined processes.

The preferred method of aligning the zone plate 5 and the refractivelens 6 is to fabricate them on the same substrate 8. As a non-limitingexample, we will assume the refractive lens 6 is fabricated from siliconas shown in FIG. 7A.

As shown in FIG. 7B, a fiducial mark 70 is added to the refractive lens6. Specifically, in the example, the fiducial 70 is added to the centerof the lens 6. In practice, the placement of multiple fiducials can leadto higher accuracy.

Since the AFO is a transmissive lens, it is often advantageous to thinthe substrate 8 to thereby fabricate an optical port 72. This reducesabsorption.

Once the substrate 8 is thinned to below 1 micrometer (um) in oneimplementation, the fiducial 70 can be imaged from the opposite sidewith a number of techniques, including but not limited to visible light.The zone plate element 5 is then fabricated in the optical port 72 suchthat it is centered at the fiducial mark 70. The accuracy of thefiducial alignment can be on the order of tens of nanometers.

Hybrid Fabrication Techniques

FIG. 8 illustrates a hybrid fabrication process for fabrication of therefractive element 6. Generally, in practice, it may prove advantageousto use a combination of different patterning techniques to fabricate thedesired profile 110, 112 of the Fresnel refractive lens 6.

In the illustrated example, a binary process, such as electron beamlithography is first used to fabricate a pattern of concentric trenches140 in the unpatterned substrate 8-1. This produces a binary-patternedsubstrate 8-2. Specifically, the binary pattern of concentric trenchesrepresents the desired step function of the desired Fresnel lens.

A suitable micromaching process, such as focused ion beam milling, issubsequently used to produce the desired profile between steps.

Specifically, substrate material for removal 142 is targeted between thetrenches 140. The targeted material 142 is then removed using thefocused ion beam 50. This yield the desired profile 112 for refractiveelement 6.

This approach has the advantage of combining the high-resolutionpatterning accuracy and depth control of electron beam lithography withthe machining capabilities of focused ion beam milling. The gradualprofile between zones is machined by focused ion beam milling, while itis extremely difficult to machine a vertical step with a great depth,which is accomplished with binary process, such as lithography.

FIG. 9 shows another embodiment of the diffractive or refractive element5, 6. Specifically, the AFO 1 can be also realized by combining afree-standing Fresnel lens and/or a free-standing zone plate lens toreduce absorptive loss.

The free-standing zone plate and Fresnel lens are realized by additionof a support structure. In the illustrate example, radial spokes 152 areincluded with the typical pattern of concentric rings 150 associatedwith the zone plate 5 or Fresnel lens 6. The spokes 152 connect andsupport the rings 150 and further connect the rings 150 to a surroundingframe 154. Often the spokes are fabricated out of the same material(e.g., silicon or copper), and with the fabrication of rings. The spokesneed not be continuous as shown but interrupted, such that only segmentsextend, at least partially in the radial direction, between eachsuccessive rings. In this way, all of the rings are connected through aseries of spoke segments.

Typically a substrate support, which is needed for the fabrication, isremoved in the final step leaving only self-supporting optical element5, 6.

Common features in the support structure of the zone plate and theFresnel lens can be used as fiducial markers to align both opticalelements in respect to each other.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims. For example, typical scanningelectron microscopes (SEM) have X-ray detectors (EDAX), which are usedto identify materials being imaged. In the fluorescence spectrometermode, the present invention is used as an element specific imagingattachment to a SEM.

1. An optical element for short wavelength radiation, the elementcomprising: concentric rings for focusing a beam of short wavelengthradiation; and segments extending at least partially radially betweenthe concentric rings to support the rings.
 2. An optical element asclaimed in claim 1, further comprising a frame extending around at leasta portion of a perimeter of the concentric rings, segments extendingbetween the rings and the frame to support the rings in the frame.
 3. Anoptical element as claimed in claim 1, wherein the optical element is azone plate lens.
 4. An optical element as claimed in claim 1, whereinthe optical element is a Fresnel refractive lens.
 5. An optical elementas claimed in claim 1, wherein the concentric rings are fabricated fromsilicon.
 6. An optical element as claimed in claim 1, wherein theconcentric rings are fabricated from copper.